The symptoms of a glaucoma attack include, nausea, vomiting, seeing halos or rainbows, headaches, redness of the eye, decreased or blurred vision and severe pain that occurs in the forehead and eye.
Glaucoma occurs when the fluid inside the eye does not drain properly and the fluid increases pressure in the eye which is called the intraocular pressure which if not drained can damage the eyesight and even lead to complete loss of eyesight.
So it's important to get Glaucoma treated as soon as possible.